Gsub and sub for replacement in Ruby - Step-by-Step Execution

Concept Flow - Gsub and sub for replacement

Start with original string

↓

Call sub or gsub method

↓

Check for first match

↓

Replace first match

↓

For gsub: Continue checking for more matches

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Replace all matches

↓

Return modified string

The program starts with a string, then uses sub to replace the first match or gsub to replace all matches, returning the new string.

Execution Sample

Ruby

text = "hello hello"
new_text_sub = text.sub("hello", "hi")
new_text_gsub = text.gsub("hello", "hi")
puts new_text_sub
puts new_text_gsub

This code replaces the first 'hello' with 'hi' using sub, and all 'hello's with 'hi' using gsub, then prints both results.

Execution Table

Step	Method Called	String Before	Match Found?	Action	String After	Output
1	sub	hello hello	Yes (first 'hello')	Replace first 'hello' with 'hi'	hi hello
2	puts new_text_sub	hi hello	N/A	Print string	hi hello	hi hello
3	gsub	hello hello	Yes (all 'hello's)	Replace all 'hello' with 'hi'	hi hi
4	puts new_text_gsub	hi hi	N/A	Print string	hi hi	hi hi
5	End	N/A	N/A	No more actions	N/A	Execution stops

💡 All replacements done and outputs printed, program ends.

Variable Tracker

Variable	Start	After sub	After gsub	Final
text	"hello hello"	"hello hello"	"hello hello"	"hello hello"
new_text_sub	nil	"hi hello"	"hi hello"	"hi hello"
new_text_gsub	nil	nil	"hi hi"	"hi hi"

Key Moments - 2 Insights

Why does sub only replace the first occurrence but gsub replaces all?

Because sub is designed to replace only the first match it finds (see execution_table step 1), while gsub continues to find and replace all matches (see step 3).

Does the original string change after sub or gsub?

No, the original string 'text' stays the same throughout (see variable_tracker), because sub and gsub return new strings without modifying the original.

Visual Quiz - 3 Questions

Look at the execution_table at step 1, what is the string after calling sub?

A "hi hello"

B "hello hi"

C "hi hi"

D "hello hello"

💡 Hint

Check the 'String After' column in execution_table row for step 1.

At which step does gsub replace all occurrences?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look for the row where Method Called is 'gsub' and action mentions replacing all matches.

If we used sub instead of gsub in step 3, what would be the output at step 4?

A "hello hello"

B "hi hello"

C "hi hi"

D "hello hi"

💡 Hint

Recall sub replaces only the first match, so only the first 'hello' changes.

Concept Snapshot

sub and gsub replace text in strings.
sub replaces only the first match.
gsub replaces all matches.
Both return new strings; original stays unchanged.
Use sub for single replacement, gsub for global.

Full Transcript

This visual execution shows how Ruby's sub and gsub methods work for replacing text in strings. Starting with the string 'hello hello', sub replaces only the first 'hello' with 'hi', resulting in 'hi hello'. Then gsub replaces all 'hello' occurrences with 'hi', resulting in 'hi hi'. The original string remains unchanged throughout. The execution table tracks each step, showing method calls, matches found, actions taken, and resulting strings. Variable tracking confirms that the original string 'text' does not change, while new variables hold the replaced strings. Key moments clarify why sub replaces only the first match and gsub replaces all, and that original strings are not modified. The quiz tests understanding of these steps and outcomes.